1. Field of the Invention
This invention relates generally to a brake system for vehicles and more particularly to a powered brake system for generating brake pressure by a pump driven by an engine.
2. Description of the Prior Art
In general, a main control valve for controlling the start and stop of a brake operation has the function of a master cylinder, and when the brake pressure cannot be generated as a result of failure of the pump, the brake pressure can be manually generated. In this system, a subcontrol valve is disposed between the main control valve and a wheel cylinder for controlling the fluid pressure to the wheel cylinder by connecting the wheel cylinder with the main control valve or a reservoir independently of the control of the main control valve. However, a subcontrol valve of the spool valve type is subject to wear and the fluid pressure leaks out from the subcontrol valve, such that there is in danger of causing a hindrance upon the manual brake operation and when the subcontrol valve is actuated in the manual brake operation, the brake operation cannot be performed due to insufficiency of fluid pressure.